A stand type of image scanner 80, such as illustrated in FIG. 10, is known as an image reading apparatus that illuminates an original with illumination light and reads light reflected from the original to read an original image. In the stand image scanner 80, illumination light (self-illumination light) 84 is illuminated onto an original from a lighting device provided to a head 82A of a stand 82, and light reflected from the original is read while a line sensor such as a line CCD provided to the head 82A of the stand 82 is being scanned. In an image reading apparatus in which illumination light is illuminated in this manner, sometimes image correction processing is required to correct color shift in an image caused by the self-illumination not illuminating the original uniformly.
As image correction processing, white balance correction is generally performed on an image captured by a digital camera. Such white balance correction corrects on the basis that the brightest portion in a captured image corresponds to a white portion in the subject, and the entire captured image is corrected by subtracting a fixed value in which the color shift in the brightest portion is taken as corresponding to a component of the color of the illumination light. Basically good correction can be achieved when such image correction processing is applied to an image read by the stand image scanner 80 as illustrated in FIG. 10 for cases in which the self-illumination illuminates the original substantial uniformly.
However correction with good precision cannot be achieved when self-illumination is non-uniform. This is because when correction is performed with a correction amount determined with reference to the brightest portion in an image, the correction amount is inappropriate for the correction amount of other portions of the image where non-uniform self-illumination occurs.
In a stand image scanner, since there is a physical limit to how far away the light source of the lighting device can be placed to try and make the self-illumination illuminate the original uniformly, it is difficult to illuminate a large size original (for example an A3 paper size original) uniformly with self-illumination.
Moreover, such a stand image scanner is not constructed with a cover to stop light from the periphery of the set original illuminating the original, such as in an ordinary photocopier. Hence not only self-illumination light but also peripheral light such as interior lighting is illuminated onto the original, and even supposing the peripheral light is illuminated substantially uniformly onto the original, combined light illuminated onto the original, of both the peripheral light and the self-illumination light combined, has a non-uniform color when the self-illumination is non-uniform. Namely the color tone of the peripheral light is stronger at locations with low illumination by the self-illumination light, and the color tone of the self-illumination light is stronger at locations where there is high illumination with the self-illumination light.
FIG. 11A illustrates an original image with the color of the original itself, and FIG. 11B illustrates an example of an original image when the original is read with the stand image scanner 80 as illustrated in FIG. 10. As illustrated in FIG. 11B, when the original is read with the stand image scanner 80, an image is produced in which the central portion of the original is brighter and the peripheral edge portions are darker due to the self-illumination illuminating the original non-uniformly. It is accordingly difficult to appropriately correct the color of the whole image even when ordinary white balance correction is performed on an image read by the stand image scanner 80.
There is also technology to correct image data when an image on an original is read using plural line sensors. Such correction is based on a received light intensity of reflected light from a standard white color sample for accurately determining shading correction coefficients for each pixel and based on a maximum value of line image data read by the line sensor.
In this technology accurate color shift correction can be achieved in an environment under self-illumination alone and without the influence of peripheral light.